you might try switching your fuel pump relay with another like relay. there could be built up resistance in the relay contacts. If you get good voltage with another relay replace the relay. If you still have reduced voltage with another relay check the voltage at the fuse. Fuse #8 in the power distribution center a 20A fuse should be hot all the time.

power goes from the battery to the power distribution center. to the fuse #8 then through a red and white wire to the relay. Goes through the relay then out on a dark green/black wire to the fuel pump. you should be able to trace voltage back to where you have the voltage loss from there